The present invention generally relates to the field of electrochemical cells. More particularly, the invention pertains to lithium rechargeable cells with long cycle life, preferably cells comprising sulfur-containing cathode materials, and to methods of making these cells.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
The need for rechargeable batteries with long cycle life, rapid charge capacity, and high energy density for devices such as mobile telephones, portable computers and other portable electronic devices continues to grow. Rechargeable batteries, such as those based on lithium metal anodes and solid electroactive sulfur-containing cathode active materials, provide one approach to meet this need. For example, U.S. Pat. Nos. 5,529,860; 5,601,947; and 5,690,702; each to Skotheim et al., and U.S. patent application Ser. No. 08/995,112 to Gorkovenko et al. of the common assignee, describe electroactive sulfur-containing cathode active materials and lithium/sulfur batteries comprising sulfur-containing cathode active materials.
However, one problem encountered in electrochemical cells based on lithium and sulfur-containing cathode active materials is limited cycle life, i.e. the number of dischargings and rechargings the battery can accept before the battery is no longer able to maintain acceptable levels of charge capacity, such as 50-80% of the initial capacity of the battery.
Electrolyte additives have been used to improve cycle life. For example, U.S. Pat. No. 5,962,171 to Boguslavsky et al. describes electrolytes containing lithium polysulfides together with 100 to 1000 ppm of water for use in lithium/sulfur electrochemical cells.
With the continued demand in portable electronic devices for rechargeable batteries with greater capacity and improved cycle life, it would be advantageous to be able to utilize a material useful in the non-aqueous electrolyte element of a lithium secondary cell which exhibits beneficial effects on cycle life and safety during the initial charge-discharge cycles of the cell and maintains its beneficial effects during the useful life of the cell, and which can be incorporated easily and reliably into the cell during assembly without significant extra cost.
It is therefore an object of the present invention to provide an additive to the non-aqueous electrolyte which is suitable for use in manufacturing secondary lithium cells and which can be conveniently added to the electrolyte.
Yet another object of the present invention is to provide such an electrolyte additive and non-aqueous electrolyte which is suitable to increase the cycle life of secondary lithium cells.
It is another object of the present invention to provide such a soluble electrolyte additive and non-aqueous electrolyte which is present and useful in the initial discharge-charge cycles of secondary lithium cells.
The present invention pertains to a secondary electrochemical cell comprising: (a) an anode comprising lithium; (b) a cathode comprising an electroactive sulfur-containing material; and (c) a non-aqueous electrolyte interposed between the anode and the cathode, wherein the electrolyte comprises: (i) one or more lithium salts; (ii) one or more non-aqueous solvents; and (iii) a cycle life enhancing amount of water. Cycle life enhancing amounts of water range from greater than 3,000 ppm by weight of the electrolyte to about 50,000 ppm by weight of the electrolyte. Preferred amounts of water range from 5,000 ppm to about 20,000 ppm by weight of the electrolyte, more preferably from 10,000 ppm to about 20,000 ppm.
Examples of electroactive sulfur-containing cathode materials include elemental sulfur and organic materials comprising both sulfur atoms and carbon atoms, which organic materials may or may not be polymeric and preferably comprise polysulfide moieties. The anode preferably comprises lithium metal. The electrolyte preferably comprises one or more non-aqueous solvents selected from the group consisting of ethers, cyclic ethers, polyethers, esters, sulfones, and sulfolanes, and one or more lithium salts selected from the group consisting of LiBr, LiI, LiSCN, LiBF4, LiPF6, LiAsF6, LiSO3CF3, LiN(SO2CF3)2, LiC(SO2CF3)3, (LiSx)zR, and Li2Sx, where x is an integer from 1 to 20, z is an integer from 1 to 3, and R is an organic group.
In one aspect of the present invention the electrolyte comprises a cycle life enhancing amount of water greater than 10xe2x88x926 moles/cm2 of the lithium surface of the cell in contact with the electrolyte.
In another aspect of the present invention a method is provided for increasing the cycle life of a secondary electrochemical cell, wherein the method comprises the steps of (a) providing an anode comprising lithium, preferably comprising lithium metal, (b) providing a cathode comprising an electroactive sulfur-containing material, as described herein, and (c) providing a non-aqueous electrolyte interposed between the anode and the cathode, wherein the electrolyte is prepared by a process comprising preparing a solution of (i) one or more lithium salts, (ii) one or more non-aqueous solvents, and (iii) a cycle life enhancing amount of water greater than 3000 ppm by weight of the electrolyte.
A further object of the present invention is to provide a method of increasing cycle life of a secondary electrochemical cell, as described above, wherein the method comprises a step of incorporating into the cell greater than 3000 ppm of water by weight of the electrolyte in the cell. The method may further comprise a step of releasing gas formed after cell assembly, and prior to sealing the cell. A further step of the method may comprise releasing the gas formed prior to the first discharge-charge cycle of the cell.